1. Field of the Invention
The present invention relates to a miniature motor having a rotor core structure which reduces cogging, and to a method for manufacturing the miniature motor.
2. Description of Related Art
There has been demand for miniature motors designed in such a manner so as to reduce cogging, for application to various fields, such as audio and video equipment. For example, a miniature motor used for driving a photo-pickup for use in an MD player or CD-ROM drive is required to reduce cogging in order to accurately control its stop position and to lower its starting voltage or current. Also, a motor for driving a printing head of a printer preferably has reduced cogging; otherwise, nonuniform printing may result.
In the case of a miniature motor having a rotor with salient poles, magnetic flux density varies greatly with rotation of the rotor. Specifically, the magnetic flux density at a certain point on the inner circumference of a stator changes greatly when a slot between core salients reaches the certain point and again changes greatly when a salient pole adjacent to the slot reaches the certain point, which results in generation of cogging. Conventionally, the slot between adjacent core salients is necessary for winding work and cannot be narrowed beyond a certain limit.
In order to facilitate such winding work, according to one known technique, ends of wing portions of a core are bent outward before winding work is performed (Japanese Patent Application Laid-Open (kokai) No. 63-178746). This technique will be described with reference to FIGS. 9 and 10. FIGS. 9 and 10 show a rotor core having three salient poles. FIG. 9 shows a state before completion of assemblyxe2x80x94windings are placed on two core salients. FIG. 10 shows a state after completion of assemblyxe2x80x94windings are placed on all of three core salients.
As shown in FIG. 9, before winding work is performed, end portions 24 and 25 of a circumferentially extending peripheral end (wing portion) of each of core salients 16, 17, and 18 are bent outward so as to expand a slot between the facing end portions 24 and 25 for convenience of winding work. After completion of winding work, as shown in FIG. 10, the bent end portions 24 and 25 of the wing portions of the core salients 16, 17, and 18 are restored to their original shapes. Thus, the peripheral portions of the core are all arranged along the circumference of the core.
Through use of this technique, winding work can be performed favorably and regularly. However it is not an easy process to bend outward the end portions 24 and 25 of the core salients 16, 17, and 18 and then restore the bent end portions 24 and 25 to their original shapes. Also, the accuracy of the technique is not sufficient for reducing cogging to a satisfactorily low level. Further, in order to plastically deform only the end portions 24 and 25, a xe2x80x9crestxe2x80x9d must be disposed on the interior side of each wing portion of the core. However, in order to enable disposition of such a xe2x80x9crestxe2x80x9d on the interior side of the wing portion, the number of turns of winding must be decreased to thereby secure a space for the rest. As a result, motor performance, such as torque, may become insufficient. In the case where the xe2x80x9crestxe2x80x9d is not employed, the thickness of the end portions 24 and 25 must be rendered sufficiently thin as compared to that of the core salients 16, 17, and 18. However, employment of the thinner end portions 24 and 25 may impair a cogging-reducing effect.
An object of the present invention is to solve the above-mentioned problems involved in the conventional miniature motor and to provide a miniature motor having a rotor core structure which can reduce cogging and which can be machined with high accuracy.
Another object of the present invention is to provide a method for manufacturing the miniature motor.
To achieve the above object, the present invention provides a miniature motor comprising: a plurality of magnets serving as stator poles and attached to an inner surface of a motor casing having a closed-bottomed cylindrical shape; a salient-pole rotor having a core mounted on a rotor shaft and windings placed on the core; and a commutator mounted on the rotor shaft. The core includes a center portion having a hole formed therein at the center for engagement with the rotor shaft, a plurality of leg portions extending integrally and radially from the center portion in equal number with the salient poles, and a plurality of wing portions, each extending integrally and symmetrically from an end of each of the leg portions. The core includes core laminations blanked from sheet steel and assembled together. The core is deformed such that the radius of a circumcircle of the core coincides with a designed final radius of curvature of outer circumferential surfaces of the salient poles of the rotor.
Preferably, the width of each slot between adjacent wing portions is equal to or less than (core diameter/number of poles)xc3x970.25.
Preferably, the width of each slot between adjacent wing portions is equal to or less than the diameter of a winding wire.
The present invention further provides a method for manufacturing a miniature motor, comprising the step of blanking core laminations from sheet steel. Each of the core laminations includes a center portion having a hole formed therein at the center for engagement with a rotor shaft, a plurality of leg portions extending integrally and radially from the center portion in equal number with the salient poles, and a plurality of wing portions, each extending integrally and symmetrically from an end of each of the leg portions. The radius of a circumcircle of each of the core laminations is greater than a designed final radius of curvature of outer circumferential surfaces of the salient poles of the rotor. The method further comprises the steps of: superposing the core laminations on each other to thereby obtain a laminated core; mounting the laminated core and a commutator on a rotor shaft; placing windings on the laminated core; and pressing the laminated core with windings from the radially outward direction by use of a forming jig having a pressing surface of a radius of curvature substantially equal to the designed final radius of curvature of outer circumferential surfaces of the salient poles of the rotor.
Thus, according to the present invention, the slot between adjacent wing portions is narrowed so as to reduce cogging of a miniature motor. Before winding work is performed, the entire wing portions of the core are opened (in distinction to the case of FIG. 9 where only wing end portions are opened), so that winding work is easy. After completion of winding work, the entire wing portions of the core are deformed, thereby adjusting the xe2x80x9cwinding shapexe2x80x9d and narrowing the gap between windings. Thus, winding density becomes higher than that of the rotor of FIG. 10.